Zinc is an essential trace element necessary for plants, animals, and microorganisms. It is the second most abundant transition metal in the human body after iron. Zinc is one of the most important cations in catalytic centers and structural cofactors of many enzymes and metalloproteins. It is an essential factor in many biological processes such as the metabolism of DNA and RNA, sign transduction, and gene expression, as well as the pathological processes in many diseases including Alzheimer's disease, epilepsy, and ischemic stroke.
In blood plasma, zinc is bound to and transported by albumin and transferrin. Since transferrin also transport iron, an excessive amount of zinc will result in insufficient absorption of iron. Although most zinc ions are tightly bound to enzymes and proteins, the physiological roles of free zinc pools in certain tissues have yet to be explored. Unlike other biological transition metal ions such as Fe2+ and Cu2+, Zn2+ is spectroscopically and magnetically silent due to its d10 electron configuration. As a result, sensitive and non-invasive fluorescence-based techniques are ideal for zinc analysis and imaging.
However, most of the fluorescent zinc sensors disclosed in the prior art are based on conventional organic luminophores, such as O'Halloran et al. (U.S. Pat. No. 7,105,680), Komatsu (U.S. Pat. No. 7,696,245), Yano et al. (U.S. Pat. No. 7,541,467), and Nagano et al. (U.S. Pat. No. 6,903,226). Most of these probes were constructed based on Rhodamine or fluorescein derivatives. These conventional organic luminophores exhibit high fluorescence in dilute solutions, but suffer from aggregation-caused quenching (ACQ) in the condensed or solid phase. When dispersed in aqueous media or fabricated into solid film, the fluorescence of conventional luminophores is often weakened or even quenched, which greatly limits their real-world applications.
Likewise, most terpyridine based metal ion sensors reported in the literature are only used in organic solvents or mixed solvents with high fractions of organic solvents. This is because these fluorophores undergo aggregation-caused self-quenching when dispersed in the aqueous media. This problem is more severe in the solid state. However, fluorophores with aggregation-induced emission (AIE) properties can overcome this problem. They are stable in water and resistant to self-quenching upon aggregation.
To make the fluorescence cation sensors work in aqueous solution, bulky alicyclics and dendritic wedges are attached to aromatic rings to obstruct the formation of aggregates. Despite the nontrivial synthetic effort, these approaches often result in undesired side effects. Still, for most organic dyes, it is difficult to distinguish Zn2+ from other metal species, including its analogue Cd2+. It is also difficult to distinguish Fe2+ from Fe3+.
Accordingly, there is a great need for the development of fluorescent sensors for metal ions with high selectivity and superior practical use, including the ability to be easily tuned by altering substituent groups and easy synthesis thereof.